1. Field of the Invention
The invention relates to the conversion of synthesis gas to hydrocarbons. More particularly, it relates to the conversion in such synthesis gas to C.sub.5.sup.+ hydrocarbons suitable for use as liquid motor fuels.
2. Description of the Prior Art
It is well known in the art that synthesis gas, i.e., hydrogen and carbon monoxide, can be converted to hydrocarbons in the presence of a variety of transition metal catalysts. Thus, certain Group VIII metals, particularly iron, cobalt, ruthenium and nickel, are known to catalyze the conversion of CO and hydrogen, also referred to as syngas, to hydrocarbons. Such metals are commonly called Fischer-Tropsch catalysts. While the use of nickel preferentially produces methane upon conversion of syngas, the use of iron, cobalt and ruthenium tends to produce hydrocarbon mixtures consisting of hydrocarbons having a larger carbon number than methane, as determined by a number of analytical means including mass spectrographic analysis of individual components and the boiling point curve method. At higher reaction temperatures, all Fischer-Tropsch catalysts tend to produce gaseous hydrocarbons, and it is readily feasible to select processing conditions to produce methane as the principal product. At lower temperatures, and usually at higher pressures, however, iron, cobalt and ruthenium produce hydrocarbon mixtures consisting of larger hydrocarbons. These products usually contain very long straight-chain hydrocarbon molecules that tend to precipitate as wax. Such wax material, boiling well beyond the boiling range of motor fuels, typically constitutes a significant fraction of the product produced in such catalytic conversion operations. Fischer-Tropsch catalysts have not been advantageously employed in the production of liquid hydrocarbon motor fuels, therefore, instead commonly producing either principally gaseous hydrocarbons, on the one hand, or hydrocarbons containing an unacceptably large amount of wax on the other. In addition, the gasoline boiling hydrocarbon fraction that has been produced has an unacceptably low octane number.
In light of such circumstances, efforts have been made to improve the performance of Fischer-Tropsch catalysts for use in various desired syngas conversions. For example, the Breck et al. patent, U.S. Pat. No. 3,013,990, discloses the use of zeolitic molecular sieves containing a Fischer-Tropsch catalyst as improved catalyst compositions. Thus, Type A, X and Y molecular sieves loaded with iron or cobalt are shown to be suitable Fischer-Tropsch hydrocarbon synthesis catalysts, as for the production of methanol from syngas. Also with respect to the conversion of syngas, Fraenkel et al., U.S. Pat. No. 4,294,725, teach that zeolites A and Y loaded with cobalt, incorporated by ion exchange and reduced in-situ with cadmium, serve as useful catalysts of the Fischer-Tropsch type. Those skilled in the art will appreciate that such catalyst materials tend to be relatively expensive and, in any event, do not produce hydrocarbon products advantageous for use as liquid motor fuels.
Efforts have also been made to improve Fischer-Tropsch catalyst performance by preparing intimate mixtures of Fischer-Tropsch metals, such as iron, with an acidic crystalline aluminosilicate, such as ZSM-5. The Chang et al. patents, U.S. Pat. No. 4,086,262, and U.S. Pat. No. 4,096,163, disclose such catalyst compositions employed in the conversion of synthesis gas to hydrocarbon mixture useful in the manufacture of heating fuels, gasoline, aromatic hydrocarbons and chemical intermediates. When it is desired to convert syngas specifically to hydrocarbons boiling in the jet fuel+diesel oil boiling range, however, such an approach is not suitable, experiencing an effective limitation at C.sub.10 carbon number as was the case using ZSM-5 in methanol conversion, as disclosed in the Owen et al. patent, U.S. Pat. No. 3,969,426.
While iron is the currently preferred Fischer-Tropsch catalyst component for use in syngas conversion operations, cobalt had originally been preferred because of its various desirable properties. Thus, cobalt has a higher level of catalytic activity in syngas conversion operations as well as a better selectivity to total motor fuels than is obtained using iron. Cobalt has certain product quality disadvantages, however, that have tended to discourage its use for syngas conversion operations. Thus, the hydrocarbon products obtained using cobalt catalysts are generally more paraffinic and waxy than the corresponding products obtained using iron as the Fischer-Tropsch catalyst. Such waxy products are much more difficult to upgrade, as by the use of a shape selective component in the Fischer-Tropsch catalyst composition in accordance with known practice, than would be a more olefinic hydrocarbon conversion product of syngas conversion operations.
It is desirable, therefore, that improvements be made in the art to enable cobalt to be more advantageously employed as a Fischer-Tropsch catalyst for syngas conversion operations. The prior art developments relating to the use of cobalt catalysts for applications other than Fischer-Tropsch catalysts do not appear relevant to the problems associated with the Fischer-Tropsch conversion of syngas to liquid motor fuels. Thus, cobalt-molybdenum catalysts supported on alumina are the commonly employed commercial hydrotreating hydrode-sulfurization) catalysts. Such catalysts, generally containing 3-5% CoO and 15% MoO.sub.3 on Al.sub.2 O.sub.3, are not particularly relevant to Fischer-Tropsch catalysts. In another area of prior art activity, molybdenum and tungsten have been employed in Fischer-Tropsch synthesis reactions.
At a 1982 Material Research Society meeting, A. Brenner reported the use of molybdenum and tungsten as Fischer-Tropsch metals. Thus, molybdenum salts were reduced at a high temperature (1000.degree. C.), and the reduced molybdenum was found to act as a very low activity Fischer-Tropsch metal. A somewhat more active catalyst can be formed by reducing molybdenum or tungsten carbonyl. Molybdic acid has also been tested as a promoter for iron Fisher-Tropsch catalysts as reported in the Fischer-Tropsch and Related Synthesis by H. Storch, N. Golumbic and R. Anderson, John Wiley & Sons, N.Y. 1951, however, the molybdic acid did not improve the activity of the iron catalyst. Storch et al also reported the testing of tungsten oxide promoters for iron Fischer-Tropsch catalysts, said promoters resulting in a shift of the product distribution toward high wax yields.
It is generally known in the art that manganese is effective in increasing the olefin content of hydrocarbon products obtained upon syngas conversion using an iron Fischer-Tropsch catalyst. Manganese is not effective, however, in producing more olefins when a cobalt Fischer-Tropsch catalyst is employed. Despite the various activities carried out in the art as indicated above, there remains a desire in the art for improvements rendering cobalt a more satisfactory Fischer-Tropsch catalyst for syngas conversion than it is at the present time. What is thus desired in the art is the development of an additive and/or an operating technique that will have a similar effect with respect to cobalt as has manganese with respect to iron catalysts.
It is an object of the invention, therefore, to provide an improved Fischer-Tropsch catalyst composition for use in the conversion of syngas to liquid motor fuels.
It is another object of the invention to provide an improved cobalt-based Fischer-Tropsch catalyst composition for said syngas conversion.
It is a further object of the invention to provide a cobalt Fischer-Tropsch syngas conversion catalyst and process capable of enhancing the olefinic content of the liquid hydrocarbon conversion products obtained.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.